Strip of steel having a variable thickness in length direction

ABSTRACT

A strip of steel having a variable thickness in its length direction with at least thicker and thinner sections, the strip having been cold rolled to form the thicker and thinner sections, one thicker and one thinner section having a length of at most a few meters, which strip has been annealed. The annealing has been performed by continuous annealing.

The invention relates to a strip of steel having a variable thickness inits length direction with at least thicker and thinner sections, thestrip having been cold rolled to form the thicker and thinner sections,one thicker and one thinner section having a length of at most a fewmetres.

A strip of steel having a variable thickness in its length direction isoften made such that the strip has a repetitive thickness variation,wherein a thicker section of the strip is followed by a thinner sectionwhich is thereafter followed by a thicker section, and this is repeatedover the length of the strip. Often the thinner sections all haveapproximately the same length, and so have the thicker sections. Onethicker and one thinner section have a length of at most a few metres.One strip can have at least a few hundred thicker and thinner sections.The thicker and thinner sections have a thickness between a few tenthsof a millimetre and a few millimetres. For special purposes, the stripis rolled into three or more different thicknesses which repeat alongthe length of the strip. Due to the fact that the strip of steel hasbeen cold rolled, between the thicker and thinner sections atransitional section will be formed in which the thickness of the stripgradually changes from the thickness of one section to the thickness ofthe following section. The length of this transitional section isdetermined by the thickness change between the sections, the rollingspeed and the speed with which the cold rolling mill can change thedistance between the rolls, to mention the most important parameters.Usually, the length of the transitional section is of the same order asthe length of the thicker and thinner sections or even shorter. Thewidth of the strip can be from a few decimetre up to about two meter.The strip can be slit into two or more strips having a reduced width,but this is not always required. Such a strip is cut into pieces whichare called tailor rolled blanks (TRBs), for instance for the automotiveindustry. The blanks thus have at least two different thicknesses overtheir lengths, as required for the purpose and place they are used in.

During the rolling of the strip of steel the thickness is considerablyreduced in the thinner portions. This results in a hardening of thesteel, such that the rolled strip cannot be used directly. The steelstrip has to be annealed to release the stresses in the strip and/or torecrystallise the strip.

Annealing of a steel strip without thickness variations can be performedeither by batch annealing or by continuous annealing. Annealing of steelstrip having a variable thickness in its length direction, however, isperformed only by batch annealing, so as to provide the same temperatureto both the thinner and the thicker sections. Batch annealing though ismore expensive than continuous annealing, and it usually has a somewhatdeteriorating effect on the strength of the steel. Due to the slowheating and cooling rate experienced in the case of batch annealing itis not attractive for all steel types, especially for steel types havinga higher strength.

It is an object of the invention to provide an improved strip of steelhaving a variable thickness in its length direction with at leastthicker and thinner sections.

It is another object of the invention to provide a strip of steel havinga variable thickness in its length direction that is more cost-efficientthan a batch annealed strip.

It is a further object of the invention to provide a strip of steel thatprovides a higher strength as compared to a batch annealed strip.

It is still another object of the invention to provide tailor rolledblanks produced from such strips of steel.

According to the invention at least one of these objects is reachedusing a strip of steel having a variable thickness in its lengthdirection with at least thicker sections and thinner sections, the striphaving been cold rolled to form the thicker and thinner sections, onethicker and one thinner section having a length of at most a few meter,which strip has been annealed, wherein the annealing is performed bycontinuous annealing.

The inventors of the present invention have observed that, contrary tothe well-known batch annealing which is the only method of annealingused for strip having a variable thickness according to the state of theart, it is nevertheless possible to continuous anneal a strip of steelhaving a variable thickness in length direction. Continuous annealinghas the advantage that it is a faster process and provides new andbetter tailor rolled blanks. Tailor rolled blanks produced usingcontinuous annealing have better mechanical properties than tailorrolled blanks produced using batch annealing having the same compositionand rolling history, such as a higher mechanical strength, and so havethe strips of steel from which such tailor rolled blanks are produced.

With batch annealing a strip having a variable thickness will havedifferent mechanical properties in the different sections because of thevariation in cold rolling reduction, whereas the annealing temperatureand heating rate will be the same in all sections. In the thinnersections a higher cold rolling reduction will produce differentmechanical properties, for instance a higher yield strength. Theadvantage of continuous annealing over batch annealing is that withcontinuous annealing the sections with a variable thickness will alsoexperience different temperatures and heating rates. In a thinnersection the temperature will reach higher values than in a thickersection. The higher annealing temperature experienced in the thinnersections will reduce the strength, which partly or completelycompensates the effect of the higher cold rolling reduction.

Preferably, the yield strength of the thicker sections is equal to orhigher than the yield strength of the thinner sections. This isadvantageous because the TRBs made from such strips are used for partsthat need to have more strength in the thicker section than in thethinner section.

According to a first preferred embodiment the steel strip is a DP, TRIPor multi phase high strength steel. These high strength steels can notbe produced using batch annealing, so continuous annealing makes the useof DP, TRIP and multi phase high strength steels possible for producingstrip having a variable thickness and the TRBs made thereof.

According to a second preferred embodiment the steel strip is a HSLAsteel or a low carbon steel. Using continuous annealing for these steeltypes provides strip having a variable thickness and TRBs made thereofthat have better mechanical properties, such as a higher yield strength.

When the strip of steel is a HSLA steel or low carbon steel, preferablyonly the thinner sections are recrystallised and the difference in yieldstrength of the thicker and thinner sections is smaller than in the sameHSLA or low carbon steel strip that has been batch annealed. Therecrystallised thinner sections reach a higher temperature due to thecontinuous annealing, compared to batch annealing, and therefore thethinner sections have for instance a higher yield strength. Thus, theyield strengths of the thicker and thinner sections have values that aremore near to each other than the corresponding values of batch annealedstrip having the same composition.

Preferably, the composition of the steel has lower values of alloyingelements than in a batch annealed HSLA or low carbon steel having thesame yield strength of the thinner sections. Since the yield strength isbetter for continuous annealed strip having a variable thickness thenfor batch annealed strip with the same composition, it is possible toprovide strip having a variable thickness with the same yield strengthas batch annealed strip, using a continuous annealed strip having lowervalues of alloying elements (which strip, when batch annealed, wouldhave a lower yield strength). Thus, the steel strip having a variablethickness is cheaper.

According to a preferred embodiment the steel has the followingcomposition in wt %:

C 0.03 to 0.08 Mn 0.1 to 1.2 Si ≦1.0 P ≦0.1 Nb ≦0.07 V ≦0.5 Ti ≦0.1the remainder being iron and inevitable impurities. This is a normalcomposition for a low carbon steel, wherein the steel can contain one ormore of the optional alloying elements Si, P, Nb, V and Ti.

According to a preferred embodiment the steel contains C, Mn, andoptionally Si, P, Nb, V, and Ti, the remainder being iron and inevitableimpurities, and is characterised by the equation:

YS≧250+225(Mn/6+Si/24)+716P+2938Nb+600V+2000Ti[MPa]

with Mn, Si, P, Nb, V, Ti in wt % and YS being the yield strength in thethinner sections of the strip. This equation shows that by usingcontinuous annealing a high yield strength can be achieved in thethinner sections of the strip with less alloying elements than would beneeded when such a strip had been batch annealed.

More preferably, the steel is characterised by the equationYS≧270+225(Mn/6+Si/24)+716P+2938Nb+600V+2000Ti [MPa]. Due to optimisedprocess conditions for the continuous annealing, the steel strip havinga variable thickness will reach the higher yield strength according tothis equation.

Preferably, the strip of steel is characterised by the equation

A80 ≧−0.05*YS+40

with A80 being the total elongation in the thinner sections of the stripand YS being the yield strength in the thinner sections of the strip.This equation shows that continuous annealed strip having a variablethickness will have product properties that are often required, that isa high total elongation combined with a high yield strength. A hightotal elongation is for instance required for stamping parts.

According to a further preferred embodiment the steel in the thinnersections has a tensile strength above 600 MPa and a yield strength below400 MPa. The steel of this strip is for instance a dual phase steel thathas been temper rolled.

More preferably the steel in the thinner sections has a tensile strengthabove 600 MPa and a yield strength below 300 MPa. The lower yieldstrength is reached by an optimised rolling schedule before and/or afterthe continuous annealing of the strip.

According to a still further preferred embodiment the steel in thethinner sections has a tensile strength above 800 MPa and a yieldstrength below 550 MPa. The steel of this strip can be a dual phasesteel as well, having a composition with higher amounts of alloyingelements, which has been temper rolled.

More preferably the steel in the thinner sections has a tensile strengthabove 800 MPa and a yield strength below 450 MPa. Here too, the loweryield strength is reached by an optimised rolling schedule before and/orafter the continuous annealing of the strip.

According to again a further preferred embodiment, the steel in thethinner sections has a tensile strength above 980 MPa and a yieldstrength below 750 MPa. Here as well, the steel can be a dual phasesteel, having a composition having still higher amounts of alloyingelements, which has been temper rolled.

More preferably the steel in the thinner sections has a tensile strengthabove 980 MPa and a yield strength below 650 MPa. Again, the lower yieldstrength is reached by an optimised rolling schedule before and/or afterthe continuous annealing of the strip.

According to a second aspect of the invention there is provided a tailorrolled blank produced from a strip of steel according to the descriptionabove. The tailor rolled blanks are cut from the strip having a variablethickness, and these tailor rolled blanks are used in the automotiveindustry, for instance.

The method according to the invention will be elucidated referring tothe figures and examples below.

FIG. 1 shows a schematic representation of a continuous annealingtime-temperature cycle;

FIG. 2 shows a schematic representation of the differences intemperature, heating and cooling rates between thin and thick sectionsof the TRB;

FIG. 3 shows a schematic representation of the use of selective heatingto adjust the differences in temperature, heating and cooling ratesbetween thin and thick sections of the TRB.

FIG. 4 shows a comparison between the yield strength measured for anumber of steel types that are batch annealed and continuous annealed.

In the FIGS. 1, 2 and 3 the temperature T is presented along thevertical axis and time t along the horizontal axis.

In FIG. 1 a typical continuous annealing time-temperature curve ispresented. The process in a continuous annealing line for steel stripoften consists of a sequential of different heating and coolingsections. As shown schematically in FIG. 1 normally a fast heatingsection (H1) is followed by a slow heating section (H2), after which thestrip reaches it maximum temperature. This maximum temperature isnormally higher than the recrystallisation temperature to ensurecomplete recrystallisation of the microstructure of the steel. In thecase of high strength steels such as DP, TRIP and multi-phasehigh-strength steels the maximum temperature must be higher than 720° C.to bring the material in the two-phase region of austenite and ferrite.The presence of austenite, which can transform into martensite, bainiteand/or retained austenite on subsequent cooling, is a prerequisite toproduce high strength steels such as DP, TRIP and multi phase highstrength steels. After realising the maximum temperature the strip canbe cooled down, which is often done in several cooling sections. In FIG.1 a slow cooling section (C1), a fast cooling section (C2) and a finalcooling section (C3) are presented. The cooling of the strip can beinterrupted for applying a metal coating process (MC), e.g. hot dipgalvanizing. After cooling of the strip temper rolling and/or othersurface and/or shape modifications can be performed in line. The wholeprocess normally takes less than 1000 seconds to complete.

In FIG. 2 the effect of continuous annealing on TRB is illustrated. Thesections with variation in thickness will show a difference in heatingand cooling rates, and as a result will follow differenttime-temperature cycles. The line S1 indicates the time-temperaturecycle for the thinner sections of the TRB, and the line S2 indicates thetime-temperature cycle for the thicker sections of the TRB. Obviouslythe exact time-temperature profile depends on many parameters, such asthe thickness profile of the strip, line speed, width of the strip,heating and cooling capacity of individual sections in the continuousannealing line. Noteworthy in FIG. 2 is the relatively large differencein temperature at the end of the fast heating section (ΔT1). Thedifference ΔT1 can in some cases reach values of more than 100° C.

The difference in temperature at maximum temperature (ΔT2) is a criticalparameter for successfully producing continuous annealed TRB. If ΔT2becomes too big the mechanical properties of the thicker and/or thinnersections become unstable. If the temperature of the thicker sectionsbecomes too low than the material is not fully recrystallised and themechanical properties, especially the elongation, are not fullydeveloped and extremely sensitive to small fluctuations of the maximumtemperature. On the other hand, if the temperature of the thinnersections becomes too high, higher than 800° C., the mechanicalproperties of especially high strength steels will deteriorate. Thedeterioration is caused by the fact that the grain size will increasewith the maximum temperature, because the fine grain size after coldrolling and recrystallisation will be eliminated by transformation. Withhigher temperatures, above 720° C., more austenite is formed and alarger fraction of the microstructure will after continuous annealingconsist of transformed material instead of recrystallised material. Thiseffect becomes especially detrimental above 800° C. because of theincrease in austenite fraction. In the case of high strength steels suchas DP, TRIP and multi-phase high-strength steels a large temperaturedifference (ΔT2) is undesirable because the mechanical properties aredirectly related to the maximum temperature, i.e. the amount ofaustenite before cooling.

The difference in temperature between the thicker en thinner sections ofthe TRB during cooling (ΔT3 or ΔT4) is also of importance. Especially ifa metal coating process like hot dip galvanizing is applied. When thestrip entering the zinc bath is too cold, the zinc will not make goodcontact with the strip surface and problems with zinc adherence andsurface quality will arise. The zinc only starts to solidify below atemperature of 420° C. When the temperature of the strip entering thezinc bath is too high, the amount of iron dissolving in the zincincreases and thus the amount of metallic dross formation in the zincbath. This can lead to a bad surface quality of the material. A highstrip temperature can cause increased alloying between the zinc layerand the substrate.

According to a preferred embodiment the temperature differences betweenthe thick en thin sections of the TRB can be reduced by selectiveheating. This is illustrated in FIG. 3. At some point during heating ofthe strip the temperature of the thicker sections is increased (H3). Thetemperature of the thicker sections can be increased to a temperaturelevel reaching that of the thin section, or even above. In this way thedifference in maximum temperature (ΔT2) can be reduced significantly.

Hereinafter four examples of the annealing tailor rolled blanks aregiven. The chemical composition of the four examples is given inTable 1. The mechanical properties, after both batch and continuousannealing, are given in Table 2.

TABLE 1 Chemical composition* C Mn P S Si Al N Nb V Cr example wt-%*10⁻³ppm wt-%*10⁻³ 1 39 276 13 6 22 27 31 14 2 42 220 13 4 25 30 30 3 51 2508 4 8 40 26 27 4 90 1700 15 5 260 45 31 550 *remainder being iron andinevitable impurities

Example 1

A steel strip is formed by hot rolling. After hot rolling, a steel striphaving a variable thickness in length direction is formed by coldrolling both the thicker sections and the thinner sections with areduction of at least 15%. As a result, both the thicker and the thinnersections will recrystallise during annealing.

When continuous annealing is performed the strength of the TRB willalways be higher than when batch annealing is applied. After continuousannealing the yield strength in the thick section is higher than de thinsection. In case of example 1 selective heating was not applied. Theline speed in the continuous line was relatively low and therefore inthis case the difference in temperature between the thin and the thicksection is relatively small.

Example 2

A steel strip is formed by hot rolling. After hot rolling, a steel striphaving a variable thickness in length direction is formed by coldrolling the thicker sections with a reduction of less than 15%, usuallyapproximately 5%, and by cold rolling the thinner sections with areduction of at least 15%, usually between 20 and 50%.

This rolling type has the advantage that in the thicker sections the hotrolled yield strength is increased by a small cold rolling reduction,which improves the yield strength, which is to a large extend retainedduring subsequent annealing. Another advantage is that cold rolling ofthe thinner sections is more easy because only the thinner sections haveto be reduced.

The yield strength of the continuous annealed strip in the thinnersections is 73 MPa higher than for the batch annealed product. Also theyield strength in the thicker sections is higher after continuousannealing. Producing TRB by only applying a large reduction to thethinner sections is a production route that has many economicaladvantages. In case of batch annealing the inhomogeneity of themechanical properties between the thinner en thicker sections is aproblem. The advantage of a high yield strength in the thicker sections,based on the mechanical properties in hot rolled condition, can not beutilised fully in case of batch annealing because the yield strength inthe thinner sections will always be much lower. In case of continuousannealing the yield strength in the thinner sections will come muchcloser to the yield strength in the thicker sections, with as result aTRB with better and more homogeneous mechanical properties.

TABLE 2 Mechanical properties Cold rolling Maximum Yield Tensile TotalThickness reduction Annealing annealing Selective strength strengthelongation Example section [mm] [%] method temp [° C.] heating [MPa][MPa] [%] remarks 1 Thin 0.6 70 Batch 640 310 395 35 comparison 1 Thick1 50 Batch 640 300 385 34 comparison 1 Thin 0.6 70 Continuous 767 no 354402 32 invention 1 Thick 1 50 Continuous 745 no 387 421 31 invention 2Thin 0.65 57 Batch 640 264 334 32 comparison 2 Thick 1.45 4 Batch 640336 389 32 comparison 2 Thin 0.65 57 Continuous 777 no 337 381 34invention 2 Thick 1.45 4 Continuous 765 no 386 427 29 invention 3 Thin0.75 70 Continuous 840 no 367 396 27 comparison 3 Thick 1.6 35Continuous 740 no 463 511 14 comparison 3 Thin 0.75 70 Continuous 825yes 372 406 27 invention 3 Thick 1.6 35 Continuous 794 yes 384 422 24invention 4 Thin 1.0 60 Continuous 820 yes 254 612 22 invention 4 Thick1.8 25 Continuous 780 yes 296 635 24 invention

Also in case of example 2 selective heating was not applied. The linespeed in the continuous line was relatively was low and therefore inthis case the difference in temperature between the thinner and thethicker section is relatively small.

Example 3

Line speed in a continuous annealing line is important economicalparameter. If line speed is low than cooling devices like gas jetcooling have to be operated at minimum capacity, outside the normaloperation modus, making it more difficult to control the striptemperature before hot dip galvanizing. Producing TRB with a normal linespeed is both for economical and practical reasons beneficial. Selectiveheating is an effective method to enable the producer to increase linespeed and at the same time improve the mechanical properties of the TRB.

In example 3, as comparison, a high strength steel is processed with aline speed of 50 m/min. It can be seen that the temperature in thethicker sections is too low to ensure complete recrystallisation. As aresult the mechanical properties are insufficient, see e.g. the lowtotal elongation of only 14%. With selective heating it is possibleincrease the temperature of thicker section to above the crystallisationtemperature. In this way it is possible to improve the mechanicalproperties of the thicker sections without raising the temperature ofthe thinner sections. The temperature of the thinner section is wellabove 800° C., raising the temperature of the thinner sections wouldlead to a deterioration of strength so selective heating is effectivemethod to produce a TRB with reasonable line speed.

Example 4

In example 4a dual phase steel is presented. Essential for producingdual phase kind of steel types is a high annealing temperature (in twophase region) and relatively high cooling rate to promote transformationfrom austenite to martensite, bainite and/or retained austenite. In caseof dual phase steel a low line speed is a disadvantage because also thecooling rate will be slow.

As with example 3 selective heating is an effective method to be able toproduce a TRB where both the thicker and the thinner sections reach asufficient high temperature, without over-heating the thinner sections,in combination with a sufficient high line speed. Chemical compositionand the mechanical properties, after continuous annealing, are given inTable 1 and Table 2. The mechanical properties are clearly in accordancewith dual phase standards, i.e. ratio between tensile strength and yieldstrength is more than 2.

FIG. 4 shows a comparison between the batch annealing and the continuousannealing for a number of low carbon steel types, of which thecomposition is given in table 3. The Yield Strength (YS) in the sectionsthat are significantly reduced by cold rolling is given on the verticalaxis, on the horizontal axis the different steel types are indicated.Such steel types are normal steel types that are produced and on themarket. From FIG. 4 it is clear that the yield strength of continuousannealed steel is significantly higher than the yield strength of thesame steel types that are batch annealed. Such improved yield strengthsare also reached in the thinner sections of a strip of steel having avariable thickness when it is continuous annealed instead of batchannealed, as elucidated in the examples above.

From FIG. 4 it also becomes apparent that for a certain yield strength abatch annealed Nb3 type steel, having a yield strength of 310 MPa, canbe replaced by a continuous annealed Nb1 steel type, which also has ayield strength of 310 MPa, or a LC steel type. This of course leads to acheaper product, because less alloying elements are needed and coldrolling is easier.

FIG. 4 contains a thick line, connecting the points of the calculatedvalues using the equation YS=250+225(Mn/6+Si/24)+716P+2938Nb+600V+2000Tifor the steel types with the composition as mentioned in table 3, in thesections that are significantly reduced by cold rolling. It will beclear that the yield strength as measured for the continuous annealedsteel types is higher than the calculated yield strength, whereas thevalues as measures on the batch annealed steel types is lower. Thecalculated values thus give a good indication of the yield stress thatwill at least be reached for a continuous annealed steel type with acertain composition.

The elements indicated in table 3 that are present below a certainamount are inevitable impurities.

TABLE 3 Typical composition (in wt %) of different steel types Steeltype C Mn Si P Nb V LC 0.045 0.22 <0.01 <0.01 <0.002 <0.002 Nb1 0.0450.25 <0.01 <0.01 0.009 <0.002 Nb2 0.06 0.25 <0.01 <0.01 0.017 <0.002 P0.06 0.5 <0.01 0.085 <0.002 <0.002 V 0.045 0.8 <0.01 <0.01 0.013 0.04Nb3 0.07 0.5 <0.01 <0.01 0.026 <0.002 Nb4 0.075 1 0.3 <0.01 0.03 <0.002

1. Strip of steel having a variable thickness in its length directionwith at least thicker sections and thinner sections such that the striphas a repetitive thickness variation, the strip having been cold rolledto form the thicker and thinner sections, one thicker and one thinnersection having a length of at most a few meters, which strip has beenannealed, wherein the strip is to be cut into pieces called tailorrolled blanks, wherein the annealing has been performed by continuousannealing.
 2. Strip of steel according to claim 1, wherein the yieldstrength of the thicker sections is equal to or higher than the yieldstrength of the thinner sections.
 3. Strip of steel according to claim1, wherein the steel strip is a Dual Phase steel, TRIP steel or multiphase high strength steel.
 4. Strip of steel according to claim 1,wherein the steel strip is a HSLA steel or a low carbon steel.
 5. Stripof steel according to claim 4, wherein only the thinner sections arerecrystallised and wherein the difference in yield strength of thethicker and thinner sections is smaller than in the same HSLA or lowcarbon steel strip that has been batch annealed.
 6. Strip of steelaccording to claim 4, wherein the composition of the steel has lowervalues of alloying elements than in a batch annealed HSLA or low carbonsteel having the same yield strength of the thinner sections.
 7. Stripof steel according to claim 4, wherein the steel has the followingcomposition in wt %: C 0.03 to 0.08 Mn 0.1 to 1.2 Si ≦1.0 P ≦0.1 Nb≦0.07 V ≦0.5 Ti ≦0.1

the remainder being iron and inevitable impurities.
 8. Strip of steelaccording to claim 7, wherein the steel is characterized by theequation: YS≧250+225(Mn/6+Si/24)+716P+2938Nb+600V+2000Ti [MPa] with Mn,Si, P, Nb, V, Ti in wt % and YS being the yield strength in the thinnersections of the strip.
 9. Strip of steel according to claim 8, whereinYS≧270+225(Mn/6+Si/24)+716P+2938Nb+600V+2000Ti [MPa].
 10. Strip of steelaccording to claim 4, whereinA80 ≧−0.05*YS+40 with A80 being the total elongation in the thinnersections of the strip and YS being the yield strength in the thinnersections of the strip.
 11. Strip of steel according to claim 1, whereinthe steel in the thinner sections has a tensile strength above 600 MPaand a yield strength below 400 MPa.
 12. Strip of steel according toclaim 11, wherein the steel in the thinner sections has a tensilestrength above 600 MPa and a yield strength below 300 MPa.
 13. Strip ofsteel according to claim 1, wherein the steel in the thinner sectionshas a tensile strength above 800 MPa and a yield strength below 550 MPa.14. Strip of steel according to claim 13, wherein the steel in thethinner sections has a tensile strength above 800 MPa and a yieldstrength below 450 MPa.
 15. Strip of steel according to claim 1, whereinthe steel in the thinner sections has a tensile strength above 980 MPaand a yield strength below 750 MPa.
 16. Strip of steel according toclaim 15, wherein the steel in the thinner sections has a tensilestrength above 980 MPa and a yield strength below 650 MPa.
 17. Tailorrolled blank produced from a strip of steel according to claim 1.